A Highly Sensitive Intensity-Modulated Optical Fiber Magnetic Field Sensor Based on the Magnetic Fluid and Multimode Interference

In many fields, such as aerospace and marine environmental monitoring, magnetic field measurement is an important link. In recent years, optical fiber magnetic field sensor has received much attention because of its advantages such as small size, electromagnetic immunity, resistance to erosion and capability of remote sensing. In that case, magnetic fluid as a kind of medium between photons and magnetic field is widely used in optical fiber magnetic field sensors. Moreover, in the process of magnetic field measurement, disturbance introduced by temperature fluctuation always happens and brings uncertainty to the sensor. Temperature is also an important parameter in production process and needs to be measured. Therefore, designing a high-sensitive optical fiber sensor for simultaneously measuring magnetic field and temperature is a valuable work. In this paper, we present a high-sensitive hollow core fiber (HCF) interferometer for simultaneously measuring magnetic field and temperature. A segment of HCF filled with alcohol is inserted into single mode fiber (SMF) with 50 m offset at two splicing joints to guide light into the wall of HCF. And then this SMF-HCF-SMF structure is packaged by a capillary tube with full magnetic fluid (MF) inside it. Since the modal field area is large enough, the silica wall can support a series of guiding modes among which modal interference occurs and the interference spectrum can be recorded by an optical spectrum analyzer. Besides thermo-optic effect and thermal expansion effect of silica itself, the RI variations caused by thermo-optic effect of alcohol and MF as well as the magneto-optic effect of MF can also cause the phase difference of the guiding modes to change, thereby rendering interference dips movable. Thus, the sensitivity of temperature or magnetic field is higher than those given in some other previous studies. In addition, it is calculated that the effective RI sensitivities of guiding modes for inside and outside liquid are different because of the peculiar non-circular symmetry structure of HCF. So there is a possibility to find two dips in interference spectrum, which are formed with different modes and have various sensitivities to the variations of temperature and magnetic field. Finally, a sensitivity matrix can be built to demodulate those two parameters simultaneously. Experimental results show that within 20-58℃, the temperature sensitivities are 112 pm/℃ and 468 pm/℃ for dip1 and dip 2 whose magnetic field sensitivities are 37 pm/Oe and 82 pm/Oe within 0-169 Oe, respectively. The proposed sensor possesses high sensitivity and good mechanical strength, and can effectively eliminate the cross disturbances between temperature and magnetic field.

A novel magnetic field sensing system based on surface plasma resonance (SPR) optical fiber sensor and filled with magnetic fluid (MF) is proposed and demonstrated for the first time. In the magnetic field SPR optical fiber sensor, SPR is excited by Ag as metallic material and MF is filled into the capillary sealed with epoxy glue, which utilizes the tunable refractive index (RI) of MF, and the transmission spectrum will change with different magnetic field intensities. The magnetic-optic effect of MF and the high RI sensitivity of optical fiber SPR sensor are utilized to enhance the sensitivity of the novel magnetic field sensor significantly. In the experiment, the performances of the magnetic field sensing system are tested by applying different measured magnetic fields. The final results indicated that a sensitivity of 303 pm/Gs is achieved. Compared with other optical fiber magnetic field sensors, the advantages of the proposed sensor in this paper are simple structure, small in size, easy to make, low cost, high sensitivity, and anti-interference.

A distributed optical fiber magnetic field sensor based on a polarization-sensitive optical frequency domain reflectometer (POFDR) is proposed. It extracts the accumulated Faraday rotation by combining the Stokes vectors and the backward Mueller matrices from the measured states of polarization (SOPs) and obtains the magnetic field component. This method avoids adjusting the input polarization during the magnetic field sensing process. It overcomes the drawback of the conventional POFDR scheme, which requires at least two sets of different input SOPs for each sensing. Finally, the aforementioned effectiveness has been experimentally verified by using a single-mode sensing fiber. The results show that the sensor has good repeatability and linearity. The measurement error of the magnetic field sensor is 19.4 mT. The measured magnetic field variations agree with the applied ones with similarities higher than 0.98.

This work presents results of analysis of the optical fiber magnetic field sensor with external conversion fabricated in the Department of Optoelectronics of the Silesian University of Technology. Such sensor can be used as optical fiber magnetic field sensors or as the optical fiber current sensors. This study is focused on possibility of using a wide range of optical wavelengths in order to extend the measuring range of this sensor.

An inline optical fiber magnetic field sensor based on hollow core fiber (HCF) wall integrated with magnetic fluid tuned by magnetic field was proposed by sandwiching a length of HCF between two single mode fibers with offset joints. Intermodal interference is present in the HCF wall, mode propagation, as well as mode field distribution within the HCF wall have been simulated, and the contrast ratio of the interference fringes in response to the variation of the applied magnetic field has been theoretically analyzed and experimentally observed. The maximum sensing sensitivity of -0.02173 dB/Oe within the linear magnetic intensity range from 150 to 300 Oe has been achieved.

A magnetic field sensor with a magnetic fluid (MF)-coated intermodal interferometer is proposed and experimentally demonstrated. The interferometer is formed by sandwiching a segment of single mode fiber (SMF) between a segment of multi-mode fiber (MMF) and a spherical structure. It can be considered as a cascade of the traditional SMF-MMF-SMF structure and MMF-SMF-sphere structure. The transmission spectral characteristics change with the variation of applied magnetic field. The experimental results exhibit that the magnetic field sensitivities for wavelength and transmission loss are 0.047 nm/mT and 0.215 dB/mT for the interference dip around 1 535.36 nm. For the interference dip around 1548.41nm, the sensitivities are 0.077 nm/mT and 0.243 dB/mT. Simultaneous measurement can be realized according to the different spectral responses.

This paper proposes a novel fibre optic magnetic field sensor based on core-offset bending structure cascaded with Fiber Bragg Grating (FBG). The sensor is fabricated by immersing a sensing structure, consisting of a single-mode fiber (SMF) with a lateral core-offset of 4 µm spliced to a single bending-ring fiber (SBF) with a bending radius of 4.5 mm and cascaded with FBG, in magnetic fluid (MF). The core-offset facilitates coupling of a portion of the core mode into the cladding, thereby enhancing the excitation of cladding modes and improving interference effects. Moreover, the cascaded FBG serves as a temperature compensation, which effectively separates temperature-induced and magnetic-field-induced variations. Experimental analysis shows that the magnetic field sensitivity of the sensor reaches 0.5117 nm/mT within the range of 0 to 27 mT, representing a 3.95-fold improvement over previously reported sensitivities. The sensor is characterized by its simple fabrication, low cost and high sensitivity.

Due to the vast area of application and reliability, fiber optic magnetic field sensors have been the subject of several studies, however, some of these application areas are submitted to temperature variations, which can hinder the sensors in monitoring the magnetic field. With this panorama, this work analyzes through computational modeling a fiber optical magnetic field sensor, using the magneto-optical Faraday effect and observing temperature effects in the sensor response. For modeling, a numerical model built in COMSOL Multiphysics is used. The results show a value for cross-sensitivity of 3.27 mT/°C in a non-optimized configuration of the sensor and of 2.47 mT/°C for an optimized configuration. A methodology for optimizing the sensor to operate in a certain temperature range, 55 to 75 °C, is also discussed. The results presented in this work show that the temperature is an important factor to be considered to improve the selectivity and to obtain the correct sensitivity of the sensor.

We present a fiber optic magnetic field sensor conceived for magnetic resonance imaging (MRI) applications. The sensor is based on the integration of fiber optic strain sensors (fiber Bragg gratings—FBGs) with a sensing material (Terfenol-D). The response of an FBG integrated with a block of Terfenol-D was preliminarily investigated by taking into account the dependence of the Terfenol-D magnetostrictive response on both the longitudinal and transversal magnetic fields, with different preloads. Based on the performed characterizations, a triaxial magnetic field sensor was designed, characterized, and fabricated. An algorithm enabling the demodulation of the magnetic field from the readout of the three FBGs was also implemented, by taking into account the interdependence among the different sensor responses. Experimental results demonstrate the ability of the triaxial sensor to measure the magnetic field. Performance assessment and critical analysis are reported as well, elucidating both the abilities and limitations of the implemented sensing configuration. Finally, as proof of principle, a sensing system constituted of 20 triaxial sensors has been fabricated and used to map the magnetic field strength distribution in an MRI diagnostic centre.

Research on fiber-optic magnetic field sensor based on surface plasmon resonance

A fiber optic magnetic field sensor is proposed and experimentally demonstrated. Pristine and Co doped ZnO nanorods of different Co concentrations (5, 10, 15 and 20 at%) were synthesized using a hydrothermal method. The synthesized nanorods were subjected to various characterization methods like X-ray diffraction (XRD), optical absorption, scanning electron microscopy, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, vibrating sample magnetometry and X-ray photoelectron spectroscopy (XPS). XRD and XPS analysis confirms that the Co ions were successfully incorporated into the Zn site of the wurtzite ZnO lattice without altering the structure. The pristine and Co doped ZnO nanorods showed remarkable changes in the M–H loop where the diamagnetic behavior of ZnO changes to paramagnetic when doped with Co. The sensor structure is composed of cladding modified fiber coated with Co doped ZnO nanorods as a sensing material. The modified cladding is proportionally sensitive to the ambient magnetic field because of the magneto-optic effect. Experimental results revealed that the sensor has an operating magnetic field range from 17 mT to 180 mT and shows a maximum sensitivity of ∼18% for 15 at% Co doped ZnO nanorods. The proposed magnetic field sensor would be attractive due to its low cost fabrication, simplicity of the sensor head preparation, high sensitivity and reproducibility.

The detection of a magnetic field by a magnetostrictive-type fiber optic magnetic field sensor (FOMS) essentially involves measuring the longitudinal strain produced in the optical fiber to which a magnetostrictive material has been bonded.1 The performance of such a FOMS is critically dependent on the coupling efficiency between the magnetostrictive material and the optical fiber. The sensing elements are commonly (a) an unjacketed fiber bonded to a metallic glass ribbon or wire,2-5 (b) an optical fiber wrapped around a metallic glass cylinder,2 and (c) an optical fiber coated with a magnetostrictive film.6,7

A magnetic-field-tuned photonics device based on magnetic fluid (MF) and a square tapered no-core fiber (NCF) sandwiched between two single-mode fibers (SMFs) has been demonstrated experimentally and theoretically. The enhanced evanescent field effect in the NCF is achieved by tapering the square NCF utilizing a fusion splicer. The spectral dependence of the proposed device on the applied magnetic-field intensity has been investigated. The results indicate that the multimode interference spectrum exhibits a blue-shift with the increment of the magnetic-field intensity. A maximal sensitivity of -18.7 pm/Oe is obtained for a magnetic field strength ranging from 25 to 450 Oe. The proposed tunable device has several advantages, including low cost, ease of fabrication, simple and compact structure, and high sensitivity. Therefore, the magnetic-field-tuned square tapered NCF is expected to find potential applications in the fields of optical fiber sensors, as well as fiber communications.

In this study, an optical fibre magnetic field sensor is experimentally demonstrated using chemical etched multimode fibre sandwiched between two single mode fibres. The obtained result of the magnetic field and refractive index response shows that the decreasing fibre diameter in the sensing region plays an important role to enhance the sensitivity of the device. The magnetic field sensing is based on a tunable refractive index (RI) property of magnetic fluid infiltrated into the capillary tube. The sensing system is verified by testing RI of different concentrations of sodium chloride (NaCl) solution before sealing in capillary tube. Experimental results show that the sensor with taper fibre diameter of 45 µm has relatively high magnetic field sensitivity of ∼0.993 dB/mT ranging from 0 to 6 mT. The proposed devices find potential applications in magnetic field sensing.

We present a novel distributed feedback fiber laser (DFB-FL) fiber laser sensor, which can measure static magnetic field through sweep acoustic wave. A fiber laser is bonded on the giant magnetostrictive material Terfenol-D (Tb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.92</sub> ) tube oriented along the longitudinal direction. When acoustic wave of different frequency is added to the fiber laser, it has the maximum response at its resonant frequency, which will vary with the external magnetic field. The measurement sensitivity of the sensor is 0.39 Hz/mT to the magnetic field. The experimental results also show that the sensor exhibits excellent linearity and directivity in response to static magnetic field.